In the manufacture of semiconductor devices, ion implantation is used to dope semiconductor wafers with impurities. Ion implanters or ion implantation systems treat semiconductor wafers with an ion beam, to produce n or p-type doped regions or to form passivation layers in the wafers during fabrication of integrated circuits. When used for doping semiconductors, the ion implantation system injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type extrinsic material wafers, whereas if p type extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted.
FIG. 1 illustrates a conventional low energy ion implantation system 10 having a terminal 12, a beamline assembly 14, and an end station 16. The terminal 12 includes an ion source 20 powered by a high voltage power supply 22 that produces and directs an ion beam 24 to the beamline assembly 14. The beamline assembly 14 consists of a beamguide 32 and a mass analyzer 26 in which a dipole magnetic field is established to pass only ions of appropriate charge-to-mass ratio to a wafer 30 in the end station 16. The ion source 20 generates positively charged ions that are extracted from the source 20 and formed into an ion beam 24, which is directed along a predetermined beam path in the beamline assembly 14 to the end station 16. The ion implantation system may include beam forming and shaping structures extending between the ion source 20 and the end station 16, which maintain the ion beam 24 and bound an elongated interior cavity or passageway through which the beam 24 is transported en route to one or more wafers or workpieces 30 supported in the end station 16. The ion beam transport passageway is typically evacuated to reduce the probability of ions being deflected from the predetermined beam path through collisions with air molecules.
The charge-to-mass ratio of an ion affects the degree to which it is accelerated both axially and transversely by electric or magnetic fields. The mass analyzer 26 is located in the beamline assembly 14 downstream of the ion source 20 and includes a mass analysis magnet creating a dipole magnetic field across the beam path in the passageway. This dipole field operates to deflect various ions in the ion beam 24 via magnetic deflection in an arcuate section of the beamline passageway, which effectively separates ions of different charge-to-mass ratios. The process of selectively separating ions of desired and undesired masses (e.g., charge-to-mass ratios) is referred to as mass analysis. Using mass analysis techniques, the beam imparted on the wafer 30 can be made very pure since ions of undesirable molecular or atomic weight will be deflected to positions away from the beam path and implantation of other than desired materials can be avoided.
Ion implantation systems (ion implanters) typically fall into one of two major categories based on different energy ranges. Low energy implanters are typically designed to provide ion beams of a few thousand electron volts (keV) up to around 80–100 keV, in which the mass analyzed beam is provided from the mass analyzer to an end station for implanting one or more wafers. High energy implanters commonly employ linear acceleration (linac) apparatus (not shown) between the mass analyzer 26 and the end station 16, so as to accelerate the mass analyzed beam 24 to higher energies, typically several hundred keV. High energy ion implantation is commonly employed for deeper implants in a semiconductor wafer 30. Conversely, high current, low energy ion beams 24 are typically employed for high dose, shallow depth ion implantation, in which case the lower energy of the ions commonly causes difficulties in maintaining convergence of the ion beam 24.
Mass analyzers 26 for high and low energy implanters are typically designed for a range of beams 24 of different mass and species. For instance, conventional mass analyzers 26 for low energy applications can provide mass separation for Boron (B11) beams of a few keV as well as Arsenic (As) beams of around 80 keV. In this case, the mass analyzer magnet bend radius is typically around 30 cm due to limitations on the ability to create high mass analyzer dipole magnetic fields to accommodate As beams at 80 keV, wherein impractical higher mass analyzer magnetic fields would be required for smaller bend radius designs. When operated for mass analysis of lower energy beams, such as 1 keV B11 beams, the same mass analyzer magnet is adjusted to provide a lower amplitude dipole field.
The mass analyzer 26 operates as a point-to-point imaging device with a corresponding focal distance. The mass analyzer 26 receives an incoming ion beam 24 along a first path or axis at a certain distance from the mass analyzer entrance, and provides the mass analyzed output beam 24 along a second axis which has a conversion point or ‘waist’ a certain distance from the exit end of the mass analyzer, at which a resolving aperture 34 is located to allow passage of the desired mass ions and to block or intercept ions of undesired mass. Thus, in implanters 10 having mass analyzers 26 designed to accommodate a wide ion beam energy range, both high and low energy beams 24 must traverse the extra distance between the ion source 20 and the mass analyzer 26 and the distance between the mass analyzer 26 and the resolving aperture 34 necessitated by the large mass analyzer bending radius, sometimes referred to as entrance and exit drift distances, respectively.
Ion beams generally, and high current beams particularly, are comprised of a high concentration of similarly charged (positive) ions which tend to cause the beam 24 to diverge away from the beam path or axis due to mutual repulsion, a space charge effect sometimes referred to as beam blowup. Beam blowup is particularly troublesome in high current, low energy applications because the high concentration of ions in the beam (high current) exaggerates the force of the mutual repulsion of the ions, while the low propagation velocity (low energy) of the ions expose them to these mutually repulsive forces for longer times than in high energy applications. Furthermore, as discussed above, the low energy beams 24 are subjected to these space charge effects over the relatively long drift distances before and after mass analysis, due to the provision of a mass analyzer 26 designed to operate over a wide energy range.
In addition, the space charge effects are more pronounced in the entrance drift distance prior to the extracted ion beam entering the mass analyzer 26, which may be as long as 30–50 cm. This is because the initially extracted beam 24 includes ions of the desired mass, as well as those of undesired mass. In the case of low energy B11 ion beams, for example, the ion source 20 creates a plasma from BF3 source gas, from which the beam ions are extracted. The extracted ion beam 24 includes the desired B11 ions, as well as other undesired constituents, such as Fluorine (F), BF1, and BF2. In this example, the B11 content is typically only about one fourth of the total ions in the initially extracted beam 24. The mass analyzer 26 and the resolving aperture 34 cooperate to restrict the transport of the undesired components, whereby the mass analyzed beam 24 that is provided to the wafer 30 consists largely of B11 ions substantially free of the undesired constituents of the beam 24 initially extracted from the source 20.
As a result of the initial mixture of ion beam constituents, however, the extracted beam current near the source 20 is about 4 times the desired (post-analysis) beam current destined for implantation at the end station. Since the extracted beam current is much larger than the post-analysis current, the extracted beam 24 is subjected to four times as much space charge (e.g., mutual repulsion) prior to entrance into the mass analyzer 26. Accordingly, there is a need for improved ion implantation apparatus and techniques for reducing the adverse effects of space charge neutralization for implanters that support a range of ion beam energies and species.